Last year's report included results on the Ca2(+) transport induced by the naturally occuring polyether carboxylic acid ionophore A23187. A second order dependence of the initial rate of Ca2(+) efflux from egg phosphatidylcholine (PC) large unilamellar vesicles (LUV) was established. An attempt was made to establish the dependence of the rate on Ca2(+) concentration by varying the amount of Ca2(+) trapped in the vesicles. This method did not give good results. Since the last progress report the rate dependence on Ca2(+) concentration was found to be first order. The study has been conducted in LUV containing the Ca2(+) sensitive dye arsenaro III (Weissman, et al, 1980) and adding varying amounts of Ca2(+) to the external medium. The initial rate of Ca2(+) was calculated from the tangent, at zero time, to the curve obtained by following the absorbance changes at 650 nm as a function of time. The data obtained at various Ca2(+) concentration are summarized.